1. Field of the Invention
The invention relates generally to the field of marine seismic data acquisition equipment. More specifically, the invention relates to structures for a marine seismic streamer, and methods for making such streamers.
2. Background Art
Marine geophysical surveying such as seismic surveying is typically performed using sensor “streamers” towed near the surface of a body of water. A streamer is in the most general sense a cable towed by a vessel. The cable has a plurality of sensors disposed thereon at spaced apart locations along the length of the cable. In the case of marine seismic surveying the sensors are typically hydrophones, but can also be any type of sensor that is responsive to the pressure in the water, or in changes therein with respect to time or may be any type of particle motion sensor or acceleration sensor known in the art. Irrespective of the type of such sensors, the sensors typically generate an electrical or optical signal that is related to the parameter being measured by the sensors. The electrical or optical signals are conducted along electrical conductors or optical fibers carried by the streamer to a recording system. The recording system is typically disposed on the vessel, but may be disposed elsewhere.
In a typical marine seismic survey, a seismic energy source is actuated at selected times, and a record, with respect to time, of the signals detected by the one or more sensors is made in the recording system. The recorded signals are later used for interpretation to infer structure of, fluid content of, and composition of rock formations in the Earth's subsurface. Structure, fluid content and mineral composition are typically inferred from characteristics of seismic energy that is reflected from subsurface acoustic impedance boundaries. One important aspect of interpretation is identifying those portions of the recorded signals that represent reflected seismic energy and those portions which represent noise.
A typical sensor streamer is assembled by coupling a plurality of streamer segments together end to end. Each segment typically includes a jacket covering the exterior, one or more strength members extending along the segment from end to end, buoyancy spacers and sensors disposed in sensor spacers at selected positions along the strength member all disposed within the jacket. Void space within the jacket not occupied by the foregoing is typically filled with a material that is introduced into the jacket in liquid form and undergoes state change to a gel like material thereafter (called buoyancy void filler or “BVF”.)
A desirable property of the material used to form the jacket is adhesiveness. Adhesiveness between the inner wall of the streamer jacket and the BVF is desirable because it can extend the lifetime of a streamer segment in case of jacket damage. Adhesion of the jacket material to the BVF can reduce salt water penetration into the interstices of the streamer segment. However, adhesiveness in the outer surface of the jacket can cause difficulties in handling a streamer cable. It is especially a disadvantage during deployment and/or in emergencies when tangled streamer(s) need to be untangled.
There is a need for a streamer jacket for use in marine streamers that is adhesive on its inner wall and substantially non-adhesive on its outer wall.